Titanium-base alloy and method of improving creep properties

ABSTRACT

A titanium-base alloy consisting by weight of about 5.5 to 6.5% aluminum, 1.7 to 2.3% tin, 0.7 to 5.0% zirconium, 0.7 to 3.0% molybdenum, 0.04 to 0.13% silicon, and the balance titanium. The addition of silicon to the otherwise known alloy produces a marked improvement in creep properties without significant detrimental effect to other properties.

United States Patent [1 1 Bomberger, Jr. et al. Sept. 3, 1974 [54]TITANIUM-BASE ALLOY AND METHOD OF 3,482,968 12/1969 Hunter 75/175.5IMPROVING CREEP PROPERTIES 3,619,184 1 1/1971 Bomberger et a1. 75/175.53,756,810 9/1973 Parris et a1 75/1755 [75] Inventors: Howard B.Bomberger, Jr.,

Canfield; Stanley R. Seagle, Warren, OTHER UBLICATIONS both of 01119AFML-TR-70-l25, Development of a 900 F. Tita- 73 Assignee: RMI Company,Niles, Ohio 22 g Alloy, Russo et y 1970. PP- & [22] Filed: Apr. 5, 1972I [21] Appl. No.: 241,286 Primary ExaminerCharles N. Lovell Attorney,Agent, or FirmWalter P. Wood [52] US. Cl 75/l75.5, 148/325, 148/133 [51]Int. Cl. C22c /00 [57] ABSTRACT [58] Field of Search 75/ 175.5;148/312i832553, A titaniunkbase alloy consisting by weight of about 5.5to 6.5% aluminum, 1.7 to 2.3% tin, 0.7 to 5.0% 56 R f Ct d zirconium,0.7 to 3.0% molybdenum, 0.04 to 0.13% 1 e erences I e silicon, and thebalance titanium. The addition of sili- UNITED STATES PATENTS con to theotherwise known alloy produces a marked 2,893,864 7/1959 Harris et al.75/1755 improvement in creep properties without significant 3,049,4258/1962 Fentiman et al... 75/1755 d t i ntal ffect to other properties,3,343,951 9/1967 Peebles 75/l75.5 3,378,368 4/1968 Minton et al. 75/17556 Claims, 3 Drawing Figures EFFECT OF SILICON CONTENT ON THE CREEPPROPERTIES I O I AVERAGE r/ue', Irours m 3 o OF Ti- 6242 MODIFICATIONSTIME FOR 0.! DEFORM4T/0N\ A7 oF 3 5 1129/ A MINIMUM ACCEPTAELE rm:SPECIFIED 40 FOR 9501- 35 KS/ AND 0./ OEFORAMT/OM 0B 30 I I RANGE OFPRESENT 20 I INVENTION I A I /0 I I I I I 1 l l I SILICON CONTENT, "/4

PATENIED ow .3 E 3 9 & iamkauu 52 R m9 Q 1 w l 3 on 539 t wmamak m Mai mIWP TITANIUM-BASE ALLOY AND METHOD OF IMPROVING CREEP PROPERTIES -whichconsists by weight of 5.5 to 6.5% aluminum, 1.7

to 2.3% tin, 0.7 to 5.0% zirconium, and 0.7 3.03.0 molybdenum, balancetitanium and unavoidable impurities. The only other elements discussedin the patent specification as being present in the alloy are oxygen,nitrogen and carbon, the last two as interstitial impurities. Silicon isnot discussed, but information available in the file history establishesthat the patentee believed he should exclude silicon altogether from thealloy for the reason that as little as 0.15% silicon has a highlydetrimental effect on notch toughness. In fact the patentee pointedlysays silicon or equivalent compoundforming elements must be omitted fromhis alloys. Alloys of compositions within the range described in thePeebles patent have many useful properties and have achieved commercialsuccess. The most common such alloy contains normally 6% aluminum, 2%tin, 4% zirconium and 2% molybdenum, and is known in the trade as the6242 alloy. Harris et a1 disclose a number of alloys of compositionapproaching the 6242 alloy, except that the alloys preferably containabout 0.1 to 2.0% silicon. Most of the alloys listed in the examples inthe patent have a silicon content of 0.5%. The apparent purpose of thepatentees is to provide alloys which have improved creep properties; thepatent does not discuss notch toughness.

I-leretofore there have been available (a) 6242 alloy to which nosilicon is added intentionally, and (b) an alloy otherwise similar to6242 to which silicon is added in an amount to produce a silicon contentof 0.2%. Only the former is presently used commercially, since thelatter showed unfavorable notch toughness properties. We have observedthat the commercial 6242 alloy usually has a residual silicon content inthe range of about 0.02 to 0.03%, even though no silicon is addedintentionally. Some of the residual silicon comes from the spongetitanium, but more is likely to come from the master alloy used tointroduce molybdenum to the sponge. Modern spectrographic techniques candetermine silicon contents in this range in the titanium-base alloys toan accuracy of about i parts per million.

An object of our invention is to provide a novel titanium-base alloywhich contains alloying elements in a range similar to that described inthe Peebles patent, but to which we add silicon in an amount to producea silicon content in a narrow critical range above the residual level,whereby our alloy exhibits marked improvement over the 6242 alloy intests for creep properties and stress rupture strength at elevatedtemperatures, without appreciable detriment to its other properties suchas tensile strength, ductility or notch toughness.

A further object is to provide a method of improving the creepproperties of an alloy otherwise similar to the Peebles alloy by addingsilicon to the alloy in an amount to produce a silicon content in anarrow critical range above the residual level but below the range usedin the prior art.

In the drawing:

FIG. 1 is a graph showing the effect of silicon additions on the notchtoughness of an alloy otherwise similar to the 6242 alloy.

FIG. 2 is a graph showing the effect of silicon additions on the creepand stress rupture strengths of an alloy otherwise similar to the 6242alloy; and

FIG. 3 is a graph showing the effect of silicon additions on the creepdeformation of an alloy otherwise similar to the 6242 alloy.

According to our invention, we formulate alloys of a composition withinthe range of the Peebles patent and we can employ similar techniques,except that we add silicon to the alloy in an amount sufficient toproduce a silicon content therein in a critical range of about 0.04 to0.13% by weight, including the residual. Like Peebles, the remainder ofthe alloy consists by weight of about 5.5 to aluminum, 1.7 to 2.3% tin,0.7 to 5.0% zirconium, and 0.7 to 3.0% molybdenum, balance titanium andunavoidable impurities. The preferred nominal analysis apart fromsilicon is similar to the commercial 6242 alloy, to wit 6% aluminum, 2%tin, 4% zirconium and 2% molybdenum, balance titanium.

The creep properties of the alloy fall off significantly as the siliconcontent is lowered nearer the residual level of 0.03%. The upper limitof silicon in our alloy is defined by the level at which variousproperties, particularly notch toughness but surprisingly also creep,start to be affected detrimentally to an unacceptable degree. Our testsshow that the creep strength is maximum at a silicon content just belowour upper limit, but the optimum silicon content of our alloy for a goodcombination of properties is about 0.08 to 0.09%. To demonstrate thesephenomena, we performed a series of tests hereinafter described.

COMPOSITION AND PREPARATION OF SPECIMENS To conduct these tests, weformulated several 25- pound ingots of a composition within the range ofthe Peebles patent, except that we added silicon in varying amounts.Table I, which follows, lists the analyses of these ingots. Heat No.20039 is actually the commercial 6242 alloy with its residual siliconcontent of 0.03% and is outside the lower limit of our invention. Theconstituents other than silicon fall within current commercialspecifications for the 6242 alloy in all the Heats except No. 21006,which is low in molybdenum.

Table I CHEMICAL COMPOSITION OF Ti-6242 MODIFICATIONS Composition, '/1

Heat No. Al Sn Zr Mn Fe I O N C Si Table I Continued CHEMICALCOMPOSITION OF Ti-6242 MODIFICATIONS Com position,

Heat No. A1 Sn Zr Mn Fe O N C Si Spec 1" Max 6.5 2.25 4.5 2.20 25 .15.04 .04 (3) Min 5.5 1.75 3.5 1.80

Spec 2 Max 6.5 2.20 4.4 2.20 25 .15 .05 .05 (3) Min 5.5 1.80 3.6 1.80

"'Pralt 6: Whitney Specification 1209D (February 15. 1971) 'Gencru1Electric Specification C50TF39-1T (March 18. I971) (31None SpecifiedTENSILE AND NOTCH TOUGHNESS TESTS We conducted tensile tests on0.250-inch gauge diameter machined specimens obtained from the barsdescribed, both at room temperature and 900 F. We tested each specimenat 0.005 in./in./min. through the yield strength and thereafter at acrosshead speed of 0.2 in./mi n. until failure. We conducted notchtoughness tests using the standard ASTM V-notch Charpy test at F. Valuesof 10 ft.-lbs. or greater in the V- notch Charpy test are generallyconsidered acceptable for titanium-base alloys. Table II, which follows,lists the results of the tensile and notch toughness tests.

Table II As Table II shows, addition of silicon produces a modestincrease in the yield strength of the alloy and a correspondingly smallbut acceptable loss in ductility. The notch toughness, as measured byimpact energy, decreases as more silicon is added, but does not decreasebelow an acceptable value as long as the silicon content remains withinthe critical limits of our invention. While our invention results in aminor loss in notch toughness, it achieves a major gain in needed creepstrength, as hereinafter demonstrated.

FIG. 1 shows graphically the effect of silicon on notch toughness. Aslong as the silicon content of the alloy does not exceed our upper limitof about 0.13%, the Charpy impact energy is not likely to drop below thegenerally acceptable minimum of 10 ft.-1bs.

CREEP AND STRESS RUPTURE TESTS We conducted creep tests on the specimensby exposing them to a stress of 35 Ksi at 950 F. We recorded both thetime at which each specimen reached 0.1% deformation and the extent ofdeformation after 100 hours. We used an optical extensometer system tomeasure the deformation. We also conducted tensile tests on thespecimens following creep exposure. Table 111, which follows, lists theresults.

TENSILE AND NOTCH TOUGHNESS PROPERTIES OF Ti-6242 MODIFICATIONS impact72F 900F Heat" Si energy UTS YS El RA UTS YS E1 RA No. ft-lbs Ksi Ksi"/1 Ksi Ksi "/1 71 k-inch bar. Heat treatment: (Beta transus 25F)lhr-AC; 1 F-8 hrAC 'Standard ASTM Charpy V-Notch test I Low Molybdenum(1.5%)

l wflm I TW'VWW' 7 Table III CREEP PROPERTIES OF ROLLED /ii-1NCH BAR OFTi6242 MODIFICATIONS Creep Results Tensile Properties I Total Time TotalDef at at 72F Heafl Si Time for def 100 hr. UTS YS El RA No. 71 hr.0.154 "/1 Ksi Ksi 11 27 20039 .030 No Exposure 162 150 16.5 45.0 114 18.21 .19 I63 148 17.0 40.3 14 .19 .18 163 148 17.0 42.9

Table 111- Continued CREEP PROPERTIES OF ROLLED "Ya-INCH BAR OF Ti-6242MODIFICATIONS Creep Results Tensile Properties Total Time Total Def atat 72F Heat Si Time for def 100 hr. UTS YS El RA No. /1 hr. 0.17! /1 KsiKsi 7! 21004 .056 No Exposure 164 151 15.0 40.3

99 25 .16 .16 166 151 16.0 36.1 98 I40 .08 .08 171 155 16.5 35.0 91 37.12 .12 162 146 15.0 34.8 20043 .080 No Exposure 167 152 14.0 35.5 98 60.12 .12 170 155 16.5 34.3 91 120 .09 .09 172 152 15.0 32.9 21005 .090 NoExposure 163 147 16.0 40.1 100 44 .13 .13 164 151 17.5 39.1 103 200 .06.05 161 141 10.0 15.5 27277 .200 No Exposure 167 154 12.5 33.9 107 43.15 .14 170 159 15.0 24.6 108 35 .13 .13 168 156 15.0 30.8

at 1,000 F using stresses of 70 Ksi and 75 Ksi. Table IV, which follows,lists the results.

Table IV I EFFECT OF SILICON ADDITIONS ON STRESS-RUPTURE 22 Ti-6242MODIFICATIONS 'loud increased to 75 Ksi at 281 hrs and failure occurredafter a combined time of 387 hrs.

loud increased to 75 Ksi at 282 hours and failure occurred after acombined time of 342 hrs WlFrig'ar {emailing Brie stfess rupturemachine.

hr-AC.

FIG. 2 shows graphically results listed in Tables [11 and IV. In curve Xwe plot the average time to reach 0.1% deformation against siliconcontent. This curve shows a well-defined peak in the creep strength whenthe alloy has a silicon content of about 0.10%, but we prefer a slightlylower silicon content because other properties commence to be affectedadversely at 0. silicon. Each point on the curve represents the averageof at least two tests. The minimum acceptable time for 0.1% deformationunder one current specification is 35 hours. The 6242 alloy, with onlyits residual silicon content, did not meet this specification, asindicated by point A on curve X. The alloy with a silicon content of0.2% barely met this specification, as indicated by point B on thecurve, but was deficient in other respects, as shown by the results ofour notch toughness tests. Curve Y, in which we plot the time forrupture at l,000 F against silicon content, rises above the scale of thegraph at our optimum silicon content, but thereafter dropsprecipitously. FIG. 3 shows graphically additional information fromTable III on the effect of silicon on creep deformation. This curve, inwhich we plot the permanent deformation at 100 hours against the siliconcontent, shows a minimum again near our optimum silicon content. Thepoints A and B in FIG. 3 correspond with the same points in FIG. 2.

The foregoing results were altogether unexpected to us. Our expectationhad been that the relation between LII the time to reach 0.1%defonnation and the silicon content would follow approximately astraight line between points A and B of FIG. 2 instead of reaching anintermediate peak. Thus our invention provides an alloy of dramaticallyimproved creep strength compared with either the 6242 al1oy or anotherwise similar alloy containing 0.2 percent silicon known previously.We maintain an acceptable level of notch toughness, but we trade alittle in this respect for a much needed increase in creep strength.

We claim:

1. A titanium-base alloy consisting by weight of about 5.5 to 6.5%aluminum, 1.7 to 2.3% tin, 0.7 to 5.0% zirconium, 0.7 to 3.0%molybdenum, silicon in an amount of at least 0.04% but less than 0.10%,and the balance titanium and unavoidable impurities, said alloy having aminimum Charpy V-notch impact energy of 10 ft.- lbs., and requiring aminimum time of 35 hours to reach 0.1% deformation when exposed to astress of 35 Ksi at 950 F.

2. An alloy as defined in claim 1 in which the silicon content is withinthe range of 0.08 to 0.09% by weight.

3. An alloy as defined in claim 1 of a weight composition approximately6% aluminum, 2% tin, 4% zirconium, 2% molybdenum, 0.08 to 0.09% silicon,balance titanium and unavoidable impurities.

4. A method of improving the creep properties of a titanium base alloywhich otherwise consists by weight of about 5.5 to 6.5% aluminum, 1.7 to2.3% tin, 0.7 to 5.0% zirconium, 0.7 to 3.0% molybdenum, balancetitanium and unavoidable impurities, and which has a residual siliconcontent of about 0.02 to 0.03%, said method comprising adding silicon tosaid alloy in an amount to produce a silicon content therein of at least0.04% but less than 0.10% including the residual, whereby the alloyattains sufficient creep strength that specimens thereof require aminimum time of 35 hours to reach 0.1% deformation when exposed to astress of 35 Ksi at 950 F, yet retain sufficient notch toughness to havea minimum Charpy V-notch impact energy of 10 ft.-lbs.-

5. A method as defined in claim 4 in which silicon is added to the alloyin an amount to produce a silicon content therein of 0.08 to 0.09%.

6. A method as defined in claim 4 in which the alloy otherwise consistsby weight of approximately 6% aluminum, 2% tin, 4% zirconium, 2%molybdenum, balance titanium and unavoidable impurities.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. Dated Spt 5,

Inventor) Howard B. Bomberger, Jr. et a1.

It is certified that error appears in the aboveidentified patent andthat said Letters Patent are hereby corrected as 'shown below:

Column 1, line 15, "0.7 5.03.05" should read Signed and sealed this 10thday of December 1974.

(SEAL) Attest:

McCOY M, GIBSON JR.. C; MARSHALL DANN Atte'sting Officer Commissioner ofPatents F ORM I o-1050 (10-69) v USCOMM-DC 60376-P69 ufs. GOVERNMENTranmuc OFFICE: 93 o

2. An alloy as defined in claim 1 in which the silicon content is withinthe range of 0.08 to 0.09% by weight.
 3. An alloy as defined in claim 1of a weight composition approximately 6% aluminum, 2% tin, 4% zirconium,2% molybdenum, 0.08 to 0.09% silicon, balance titanium and unavoidableimpurities.
 4. A method of improving the creep properties of a titaniumbase alloy which otherwise consists by weight of about 5.5 to 6.5%aluminum, 1.7 to 2.3% tin, 0.7 to 5.0% zirconium, 0.7 to 3.0%molybdenum, balance titanium and unavoidable impurities, and which has aresidual silicon content of about 0.02 to 0.03%, said method comprisingadding silicon to said alloy in an amount to produce a silicon contenttherein of at least 0.04% but less than 0.10% including the residual,whereby the alloy attains sufficient creep strength that specimensthereof require a minimum time of 35 hours to reach 0.1% deformationwhen exposed to a stress of 35 Ksi at 950* F, yet retain sufficientnotch toughness to have a minimum Charpy V-notch impact energy of 10ft.-lbs.
 5. A method as defined in claim 4 in which silicon is added tothe alloy in an amount to produce a silicon content therein of 0.08 to0.09%.
 6. A method as defined in claim 4 in which the alloy otherwiseconsists by weight of approximately 6% aluminum, 2% tin, 4% zirconium,2% molybdenum, balance titanium and unavoidable impurities.